1. Field of the Invention
The present invention relates to surface zener diodes and, more particularly, to an apparatus and method for reducing surface zener drift in a zener diode.
2. Description of the Related Art
A zener diode is a pn junction that has a reverse breakdown voltage that defines two distinctly different regions of reverse-bias operation. When the pn junction is reverse biased, but the reverse-biased voltage is less than the reverse breakdown voltage, only a small leakage current flows through the junction.
On the other hand, when the reverse-biased voltage is increased to exceed the reverse breakdown voltage, a large breakdown current flows through the junction. Zener diodes are commonly used to provide a stable reference voltage by permanently biasing the diode to have a reverse-biased voltage that is greater than the reverse breakdown voltage.
FIG. 1 shows a cross-sectional view that illustrates a conventional surface zener diode 100. As shown in FIG. 1, diode 100, which is formed in a nxe2x88x92 semiconductor material 108, includes a n+ region 110 and an adjacent and overlapping p region 112 that are formed in nxe2x88x92 material 108.
In operation, when a first voltage is placed on n+ region 110 and a lower second voltage is placed on p region 112 such that the reverse biased voltage is less than the reverse breakdown voltage, only a small leakage current flows through the junction.
However, when the voltage on n+ region 110 is increased to exceed the reverse breakdown voltage of diode 100, a large breakdown current flows through the junction. When the breakdown current flow is primarily a lateral flow across the junction at the surface of p region 112, diode 100 is often referred to as a lateral diode or a surface zener diode.
One surface zener diode problem, known as surface zener drift, occurs when the reverse breakdown voltage of the diode drifts over time. Since the active junction formed between n+ region 110 and p region 112 is primarily a surface junction, it is more susceptible to the presence of hydrogen. The presence of hydrogen can significantly increase the reverse breakdown voltage characteristics of the diode, resulting in an observed voltage drift over time. When the diode is used as a stable reference voltage (the diode is permanently biased to have a reverse-biased voltage that is greater than the reverse breakdown voltage), the drift can lead to degraded circuit operation and potential device failure.
Thus, there is a need for a method of forming a surface zener diode that minimizes drift in the reverse breakdown voltage over time.
The present invention provides a method of forming a surface zener diode that substantially reduces drift in the reverse breakdown voltage of the diode. A semiconductor structure formed in accordance with the present invention is formed in a semiconductor material of a first conductivity type. The semiconductor structure includes a first region of the first conductivity type that is formed in the semiconductor material. The first region has a dopant concentration that is greater than the dopant concentration of the semiconductor material.
The semiconductor structure also includes a second region of a second conductivity type that is formed in the semiconductor material to adjoin the first region, and a layer of isolation material that is formed on the semiconductor material.
The semiconductor structure also includes a conductive contact that is formed through the layer of isolation material to make an electrical contact with the first region. In addition, a first metal trace is formed over the layer of isolation material and the conductive contact.
The semiconductor structure additionally includes a layer of insulation material that is formed on the first metal trace, and a conductive via that is formed through the layer of insulation material to make an electrical contact with the first metal trace. Further, the structure includes a second metal trace that is formed on the layer of insulation material and the conductive via to make an electrical contact with the conductive via. In addition, a layer of passivation material is formed over the second metal trace. The layer of passivation material, in turn, includes nitride.
In accordance with the present invention, the semiconductor structure also includes a titanium protection layer that is formed over the layer of isolation material and the conductive contact, and below the layer of passivation material. The titanium protection layer can be formed on the isolation layer and the conductive contact under the first metal trace, or on and over the second metal trace. Alternately, the titanium protection layer can be formed on the insulation layer and the conductive via under the second metal trace, or on and over the second metal trace.
The present invention also includes a method for forming a semiconductor structure in a semiconductor material of a first conductivity type. The semiconductor structure includes a first region of the first conductivity type that is formed in the semiconductor material. The first region has a dopant concentration that is greater than the dopant concentration of the semiconductor material.
The semiconductor structure also includes a second region of a second conductivity type that is formed in the semiconductor material to adjoin the first region, and a layer of isolation material that is formed on the semiconductor material. The semiconductor structure also includes a conductive contact that is formed through the layer of isolation material to make an electrical contact with the first region.
The method of the present invention includes the steps of forming a first metal trace over the layer of isolation material and the conductive contact, and forming a layer of insulation material on the first metal trace. The method also includes the step of forming a conductive via through the layer of insulation material to make an electrical contact with the first metal trace.
The method further includes the step of forming a second metal trace on the layer of insulation material and the conductive via to make an electrical contact with the conductive via. The method additionally includes the steps of forming a layer of passivation material over the second metal trace, and forming a titanium protection layer over the layer of isolation material and the conductive contact, and below the layer of passivation material. The layer of passivation material includes nitride.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.